Growth and seasonal changes in energy content of young Baltic Sea herring (Clupea harengusL.)

Growth rates, hatch-date distributions and seasonal changes in energy content of larval and juvenile herring (Clupea harengus), the major Baltic Sea zooplanktivore, were studied in a coastal area. The first microstructure (ring) shown in the otoliths (sagittae) was formed at the end of the yolk-sac stage. The following rings occurred regularly and showed a pattern corresponding to daily growth rings, as described in the literature, and we used these rings to age the fish. The average growth rate was estimated at 0.37 mm d "1 and 0.58 mm d "1 for larval and metamorphosed herring, respectively. The overall growth for all young stages could be described with a fourth order polynomial relationship between length and age. Estimated hatching dates were distributed between March and June. This is earlier than normally assumed for the northern Baltic Sea and was probably caused by an early and warm spring in 1993. Our data indicate the highest survival for herring spawned from end of April to the middle of May. In terms of seasonal and yearly fluctuations in body composition, there was an increase in energy content from 2200 to 2800 J g "1 wet weight and 2900 to 4800 J g "1 wet weight for larval and metamorphosed herring, respectively, with nearly constant energy content during metamorphosis. ? 1996 International Council for the Exploration of the Sea